Influence of the molar concentration of phosphoric acid solution on the properties of metakaolin-phosphate-based geopolymer cements

Six different phosphoric acid solutions with molar concentrations 4, 6, 8, 10, 12 and 14 M were prepared by dilution of commercial phosphoric acid in distilled water. The obtained phosphoric acid solutions were used for producing metakaolin-phosphate-based geopolymer cements. These geopolymer cements were obtained by adding each fresh prepared phosphoric acid solution to metakaolin with a mass ratio phosphoric acid solution/metakaolin = 0.80. It was observed in the infrared spectra of phosphoric acid solutions 8, 10, 12 and 14 M, the absorption bands at 875-879 and 949-972 cm(-1) which are assigned to H2PO4-. The SEM images of geopolymer cements exhibit the presence of berlinite (AIPO(4)) which is dispersed in the matrix and contributes to enhancing the strength, whereas the geopolymer cements from phosphoric acid solution with 14 M are heterogeneous microstructures. The compressive strength of phosphate-based geopolymer cements was between 36.4 and 93.8 MPa; it increased with increasing the concentration of phosphoric acid solutions up to 10 M. It can be concluded that the most convenient concentration of phosphoric acid solution required for producing phosphate based geopolymer cements with good mechanical properties is around 10 M.